Misfire? Running rough around 50 mph and 12 to 1500 rpm?

My 2001, 150000 miles, Sub just started running a bit rough, possibly a misfire, but not sure? Around 45 to 50 mph, 1200 to 1500 rpm, most pronounced going uphill, engine seems to skip, run rough, misfire?, If i downshift it doesn't seem to do it? Normal idle and low speed, smooth running. There isn't any check engine light on. I thought about plugs and new wires, but some of the stuff I have read here doesn't really back that up as a solution. Possibly and O2 sensor although I thought I would get a check engine light for something.

Does anyone have any thoughts or recommendations before spending a $1000 with mechanic checking things of the list. Thanks in advance for any help!!